A solid website experience is the result of a joint effort between a designer and a frontend engineer. Each individual brings their own expertise to the table. Through collaboration, they build something better than either could have built in isolation. As frontend engineers, we are accustomed to taking designs, implementing them, and then moving on. However, there are skills that engineers possess, that designers don't, that can bring user experiences to the next level.
Designers operate in an artificial environment. There are no network requests. Screen sizes tend to be fixed. Performance is not a concern. This is not the fault of the designer. Figma is not Google Chrome. Figma is far removed from the real world of slow internet connections and devices with limited processing power.
Frontend engineers live the opposite experience. They are concerned with interactions within the browser. They contemplate network requests, latency, caching, memoizing, rendering, bundling, lazy loading, browser behavior, and so much more. While these terms are technically focused, their end goal is to improve the user experience.